So-called mono-sheet instant photographs in general are used in the form of a film unit comprising a light-sensitive sheet comprising a support having thereon at least one light-sensitive silver halide emulsion layer, a cover sheet comprising a different support having thereon at least an acid polymer layer for neutralization, and a container, which can be ruptured by pressure, containing an alkaline developer and interposed between two sheets. The side of the light-sensitive sheet not having the light-sensitive silver halide emulsion layer and the side of the cover sheet not having the acid polymer layer become outermost layers of the film unit and therefore, a back layer containing a slipping agent for achieving good delivery from the camera or an ultraviolet absorbent for preventing deterioration of the image due to ultraviolet ray, is often provided thereon. The binder of this back layer is preferably a hydrophobic polymer in view of water resistance because the layer is the outermost face. On the other hand, the support on which the acetylcellulose is coated is, in the case of a photographic light-sensitive material, generally subjected to gelatin undercoating. However, when the acetylcellulose film is merely coated on the gelatin-undercoated support, the adhesion between layers are poor and the layer coated is easily peeled off.
Accordingly, a method of improving the interlayer adhesion is being attempted, for example, by replacing the gelatin undercoat by nitrocellulose undercoat having physicochemical properties analogous to those of the acetylcellulose film, however, this requires a special process and is not preferred in view of the cost.
Further, a technique of providing a so-called primer layer between layers so as to improve the interlayer adhesion is disclosed in U.S. Pat. Nos. 4,297,432 and 4,297,195, European Patent Publication 45692A2, JP-A-57-58148 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-57-58146 and JP-A-59-65842, however, one layer increases in the layer structure and this is either not preferred in view of the cost.